zod/v4: should be using `.encode()` for serializerCompiler

[racinette]

Now that codecs were introduced, it's only logical to support the bidi decode <-> encode semantics. The serializer compiler implementation should rather look like this:

app.setSerializerCompiler(({ schema, method, url }) => (data) => {
      if (!(schema instanceof z.ZodType)) {
        throw new Error(`${method} ${url} has no zod schema: ${JSON.stringify(schema)}`);
      }
      const encoded = schema.encode(data);
      return JSON.stringify(encoded);
    });

Except .transform calls will throw with this approach, since transform is a decode-only operation.